The present invention relates to a screw rotor for a screw pump device such as a screw compressor and a screw vacuum pump.
FIGS. 5 to 11 show a known screw compressor including a casing 13 having a suction opening 11 at one end and a discharge opening 12 at the other end, and a pair of female and male screw rotors 14 and 15 (which will be hereinafter referred simply to as rotors) rotatably mounted in the casing 13 and meshing with each other. The male rotor 15 is driven, and the female rotor 14 meshing with the male rotor 15 is rotated by the male rotor 15 in a direction as depicted by an arrow. Gas sucked from the suction opening 11 is enclosed among tooth spaces A of the female and male rotors 14 and 15 and the casing 13. While being compressed during rotation of the rotors 14 and 15, the gas is discharged from a discharge port 16 (see FIGS. 6 to 11) to the discharge opening 12.
FIGS. 6 to 11 show a time dependent change in the tooth spaces A of both the rotors 14 and 15 as viewed from an end surface on the side of the discharge opening 12. FIG. 6 shows a reference condition where a rotational angle .alpha. of the male rotor 15 is 0.degree., and FIGS. 7 to 11 show the conditions where the rotational angles .alpha. are 12.degree., 24.degree., 36.degree., 43.2.degree. and 60.degree., respectively.
In the conditions shown in FIGS. 6 to 10, the tooth space A has a portion opening into the discharge port 16, and is gradually reduced in volume to discharge the compressed gas contained therein to the discharge port 16. On the other hand, in the condition shown in FIG. 11, the tooth space A is completely isolated from the discharge port 16 to define an enclosed space. Under such an enclosed condition, the volume of the enclosed space is reduced toward zero, causing a very high gas pressure in the enclosed space. As a result, both the rotors chatter during rotation to cause the generation of abnormal noise and abnormal vibration.
Such a problem is caused by the fact that the rotor tooth has an unsymmetrical tooth profile. While there have been proposed various unsymmetrical tooth profiles in Japanese Patent Publication Nos. 60-35557 and 60-42359 and Japanese Patent Laid-open Publication No. 60-153486, the above-mentioned problem occurs in any of the proposed unsymmetrical tooth profiles.
The cause of this problem will now be described in more detail. In the conditions shown in FIGS. 6 to 10, the rotor teeth meshing with each other receive a force from the compressed gas in such a direction counter to the rotational direction of the female rotor 14. Accordingly, a torque of the male rotor 15 is transmitted to the female rotor 14 in such a manner that a trailing tooth surface of the female rotor 14 is urged by a leading tooth surface of the male rotor 15. In contrast, in the condition shown in FIG. 11, since the gas pressure in the enclosed tooth space A is abnormally high, the rotor tooth of the female rotor 14 receives a torque (negative torque) which functions to rotate the female rotor 14 in its rotational direction contrary to the case of FIGS. 6 to 10. Accordingly, the female rotor 14 is rotated in such a manner that a leading tooth surface of the female rotor 14 contacts a trailing tooth surface of the male rotor 15. Upon transition from the condition of FIG. 10 to the condition of FIG. 11, the tooth surfaces of both the rotors strike against each other to cause the generation of abnormal noise.
Particularly in case of an oil-cooling type device, a liquid oil is enclosed in the tooth space A under the condition shown in FIG. 11. Therefore, the above problem is remarkable, and there is a possibility that the rotors will be broken occasionally.
In order to prevent this defect, there has been proposed a device for eliminating the abnormal high pressure in the enclosed space, wherein a recess is formed on the end surface of the casing 13 on the discharge opening side facing the rotor accommodating chamber, so as to be communicated with the space on the suction opening side, so that the gas and/or oil contained in the enclosed space may be relieved through the recess to the space on the suction opening side (see Japanese Patent Publication No. 62-358).
Further, there has been proposed a tooth profile intended to prevent the generation of the negative torque (see Japanese Patent Laid-open Publication No. 58-113595).
However, in the device disclosed in Japanese Patent Publication No. 62-358, since the high-pressure gas in the enclosed space is relieved to the space on the suction opening side so as to prevent the generation of the negative torque, volumetric efficiency and adiabatic efficiency are deteriorated.
In the device disclosed in Japanese Patent Laid-open Publication No. 58-113595, there is a possibility of the negative torque being generated because of any factors such as a shape of the discharge port and a rotating speed of the rotors. Thus, there remain indefinite factors as to the generation of the negative torque in this prior art device.
In any case, these prior art devices are so designed as to aim to reduce or eliminate the negative torque.